1. Technical Field
The present invention relates to a mixer circuit, a communication device, and electronic equipment.
2. Related Art
Ultra Wide Band (UWB) communication is a kind of communication method for performing high-speed large-capacity data communication using a very wide frequency band. Although as the communication method using wide-band signals, there are cited a spread spectrum system and orthogonal frequency division multiplexing (OFDM) in the related art, the UWB is a further wide-band communication method using pulses with very short period of time, and is also called impulse radio (IR) communication. Hereinafter, the communication method will be described as a UWB-IR method or simply an IR method. In the IR method, modulation and demodulation can be performed only by time-base operations in contrast to the modulation in the related art, and simplification and reduction in power consumption of the circuit are considered to be expected (see specifications of U.S. Pat. No. 6,421,389 (Document 1), USP-A1-20030108133 (Document 2), and USP-A1-20010033576 (Document 3)).
Firstly, FIG. 15A shows a typical block diagram of the UWB communication device using the IR method in the related art, and FIGS. 15B and 15C show timing charts for explaining an outline of the operation thereof. The operation and the principle thereof will briefly be explained using these drawings.
The data to be transmitted is input in a terminal 1201. A pulse generation circuit 1202 generates wide band pulses. On this occasion, the pulse generation circuit 1202 receives the transmission signal input to the terminal 1201, and executes predetermined modulation on the pulses to be generated. As the modulation method, Pulse Position Modulation (PPM) for shifting the generation positions of the generated pulses, Bi-phase Modulation (BPM) for inverting the polarities of the generated pulses, and so on are often used. FIG. 15B shows waveforms of the PPM, and FIG. 15C shows waveforms of the BPM. In the drawings, solid lines and broken lines denote bit 1 and bit 0, respectively. The pulses thus generated and modulated are emitted to space via a transmitting antenna 1203.
Then, an outline of a typical receiving device of the related art will be explained. The signal received by a receiving antenna 1204 is amplified by a low-noise amplifier (LNA) 1205, and then transmitted to a mixer circuit 1206. On this occasion, an equalization process or the like for eliminating distortion caused in the transmission channel is executed if necessary. As an example of the distortion, there can be cited distortion caused by multipath, frequency shift caused by a Doppler effect, and so on.
The received signal amplified by the LNA 1205 is transmitted to the mixer circuit 1206, and multiplied by a template pulse generated by a template pulse generation circuit 1208. The mixer circuit 1206 is a kind of multiplication circuit, and outputs the multiplication value of two signals (the received signal and the template pulse in this case). The signal the mixer circuit 1206 has output is smoothed by an integration circuit 1210, and bit information transmitted from the result is discriminated by a discrimination circuit 1212, and then output from a terminal 1213 as a demodulated output. In other words, the mixer circuit 1206 and the integration circuit 1210 constitute a correlator, and calculate correlation between the received signal and the template pulse. The discrimination circuit 1212 performs determination (demodulation) of the transmitted signal based on the calculation result of the correlation.
Based on the timing charts shown in FIGS. 15B and 15C, an outline of the operation of the UWB communication device using the IR method of the related art is illustrated.
The explanation will be started from the operation of PPM along the chart shown in FIG. 15B. The received signal b received by the receiving antenna 1204 and then amplified by the LNA 1205 becomes to have a waveform shown in FIG. 15B. In the following explanations, it is assumed that the solid lines represent the case in which bit 1 is transmitted thereto, and the broken lines represent the case in which bit 0 is transmitted thereto. The template pulse generation circuit 1208 generates a template pulse c corresponding to bit 1 as shown in FIG. 15B. The mixer circuit 1206 multiplies the received signal b by the template pulse c to output a multiplication resultant signal e. The multiplication resultant signal e is integrated by the integration circuit 1210 to eliminate the high frequency component and to input to the discrimination circuit 1212, and then determined as the transmitted information in the discrimination circuit 1212 due to the magnitude of the correlation value.
Although the case of detecting the signal corresponding to bit 1 in the above description is explained, in the case of detecting the signal corresponding to bit 0, the template pulse generation circuit 1208 generates a template pulse d for bit 0 instead of the template pulse c for bit 1 to multiply the received signal b by the template pulse d, and then the mixer circuit 1206 multiplies the received signal b by the template pulse d to output a multiplication resultant signal f.
As described above, the receiving method for calculating the correlation with the template pulse for performing demodulation is generally called a synchronous detection method. In the synchronous detection method, it is required that the template pulse and the received signal are exactly the same in timing. In the example of the related art cited here, synchronization tracking is performed by controlling the template pulse generation timing of the template pulse generation circuit 1208 so that the correlation value becomes the maximum based on the determination result of the discrimination circuit 1212. Although this operation is not easy in general, it is thought to have become possible to perform the operation in a stable manner even at a high frequency due to the recent advancement of the device technology and the digital signal processing technology, by taking advantage thereof.
FIG. 15C is a chart for explaining an outline of the operation of the UWB transmission device using the IR method in the related art in the case of BPM. The received signal g received by the receiving antenna 1204 and then amplified by the LNA 1205 is multiplied by a template pulse h, which is generated by the template pulse generation circuit 1208, by the mixer circuit 1206 to form a multiplication resultant signal i. By eliminating the high frequency component from the multiplication resultant signal i by the integration circuit 1210, and determining whether the multiplication resultant signal i is positive or negative by the discrimination circuit 1212, whether the transmitted bit information is 1 or 0 can be determined. It is possible to use a low pass filter (LPF) as the integration circuit 1210, because it is equivalent to substantially obtain the correlation.
In the UWB communication with the IR method, the signal is intermittent, but is not continuous as in the narrow band communication of the related art. Therefore, it is known that power consumption of the entire receiving device can significantly be reduced by supplying the circuits of the receiver with power only when the received signal exists (or it is expected that the signal can be received), and blocking the circuits when no signal exists (see e.g., A CMOS IMPULSE RADIO ULTRA-WIDEBAND TRANSCEIVER FOR 1 Mb/s DATA COMMUNICATION AND ±2.5 cm RANGE FINDINGS T. Terada et al., 2005 Symposium on VLSI Circuits Digest of Technical Papers, pp. 30-33 (Document 4)).
In FIG. 15A, as the pulse generation circuit 1202 and the template pulse generation circuit 1208, the circuits described in the Document 4 and “A Low-Power Template Generator for Coherent Impulse-Radio Ultra Wide-Band Receivers, Jose Luis et al., Proceedings IEEE ICUWB, 2006 pp. 97-102” (Document 5) can be used. These circuits can be constituted by digital circuits, and designed so as to consume power only when the signal exists and not to consume power when no signal exists using complementary metal oxide semiconductor (CMOS). Particularly in the Document 5, it is possible to generate short pulses with a high frequency near the limit of the semiconductor elements constituting the circuit, and it is possible to generate pulses with such an extremely wide band as to be applied to UWB, namely pulses with short width. Moreover, it is possible to make the power consumption when no signal is generated, namely in the standby state, extremely low.
Further, in for example the Document 4 and “A 0.18 μm CMOS Switchable Low-Power LNA for Impulse Radio Ultra Wide-Band Receivers, E. Barajas et al., Proceedings IEEE ICUWB, 2006” (Document 6), there is introduced a low-noise amplifier 1205, which operates only in the case in which a signal exists, and has extremely small power consumption in the other cases.
FIG. 16 shows a low-noise amplifier 1300 of the Document 6. The low-noise amplifier 1300 uses two identical circuits 1311, 1312 for amplifying a differential signal. In the circuit 1311, the transistors 1301, 1302 form an amplifier having a grounded-source transistor 1301 and a grounded-gate transistor 1302 coupled in series in a manner called a cascade arrangement, which is often used as a low-noise amplifier.
A differential signal RF+ is applied to a terminal 1308, and then applied to the gate of the grounded-source transistor 1301 via a matching circuit composed of a capacitor 1305 and an inductor 1304. The signal amplified by the transistor 1301 is applied to the transistor 1302 with the gate grounded (Bias2) via a terminal 1306 to be amplified, and then a signal IF+ is taken out therefrom by the voltage drop caused by an inductor 1303.
A terminal 1309 is a terminal for providing bias (Bias1) to the gate of the grounded-source transistor 1301, and applies the bias (Bias1) thereto via a resistor 1310. Further, the terminal 1306 is for providing the bias (Bias2) to the gate of the transistor 1302, and the current flowing through the amplifier (the transistors 1301, 1302) can be controlled by controlling the bias (Bias2). Specifically, an appropriate bias voltage (Bias2) is applied when operating the amplifier, and the voltage value is minimized (e.g., the ground potential) when the operation of the amplifier is not required. On this occasion, since the current flowing through the paths of the inductor 1303 and the transistors 1301, 1302 becomes zero, it is possible to stop the operation by minimizing the potential (Bias2) applied to the terminal 1306 thereby making the circuit current zero when the operation of the amplifier is not required. In the UWB-IR, it is possible to reduce the power consumption of the low-noise amplifier by minimizing the potential of the terminal 1306 when no signal exists.
As the mixer circuit 1206 (FIG. 15A), a double balanced mixer circuit (also called a Gilbert circuit) often used in general can be used, and in the case of paying special attention to the power consumption, a passive mixer using a switching element such as a CMOS transistor can also be used.
It has been described above that in the communication device using the UWB-IR method shown in FIG. 15A, the power consumption of the entire circuit can be reduced by the technology of intermittent operation in which the circuit is made active only when a signal exists. It is obvious that each of the circuit elements constituting the communication device is required to operate so fast as to deal with the high-frequency wide-band signal of UWB-IR.
In particular, for the pulse generation circuit 1202, the template pulse generation circuit 1208, and the low-noise amplifier 1205, the circuits provided with high-speed operation performance and the intermittent operation functions are designed. However, no circuit suitable for such an operation exists in the mixer circuit (the multiplication circuit) 1206. The double balanced mixer circuit in the related art is not capable of such an intermittent operation as described above, and the passive mixer consuming no power has a problem of small conversion gain.
Further, in the related art, there is a problem that the low-noise amplifier, the mixer, and the template pulse generation circuit as essential constituents of the UWB-IR communication device, particularly in the receiving device thereof, are required to be designed individually, and then structured in combination.